The present invention relates generally to tonometers of the aneroid type and, more particularly, to an improved aneroid tonometer wherein instrument error due to temperature variations is substantially eliminated.
The quality of measuring instruments in general, including tonometers, obviously is a function of the measuring precision of the instrument. In this connection, the term "measuring precision" is used not only to refer to the resolution of the instrument but, also, is understood to also comprise various factors such as the reliability and stability of the measurement value.
However, in the case of precision measuring instruments, such as the tonometer, it is known that the inner mechanism of the instrument is subject to being affected by environmental conditions, such as changes in temperature, humidity and atmospheric pressure, such changes in ambient conditions often causing serious errors in the value measured by the instrument. Thus, generally, precision measuring instruments are often subject to question regarding the reliability, stability, etc., of the measured value due to such environmental conditions.
The aneroid tonometer, the subject matter of the present invention, is particularly susceptible to instrumental error resulting from changes in ambient temperature since the respective components of the tonometer usually have different coefficients of thermal expansion. Such errors in measured values in aneroid tonometers is a well known problem and an urgent demand exists for the provision of an improved aneroid tonometer which is substantially free from instrumental error resulting from ambient temperature changes.
The construction of a well known prior art tonometer is illustrated in FIG. 1. Referring to FIG. 1, the prior art tonometer includes a nozzle portion 18 located in a lower portion of the instrument. The nozzle portion 18 is adapted to communicate with the object pressure to be measured. The object pressure is transmitted through the nozzle portion 18 to a bellows 19. For example, where the tonometer functions as a barometer, the object pressure comprises atmospheric pressure. The bellows 19 is expanded or contracted depending upon the pressure communicated thereto and thereby a pointer 16 is swung by a rod 14 and an amplifying mechanism 15 by an amount which corresponds to the change in expansion of the bellows 19.
The rod 14 has its lower end loosely inserted into a small diameter tubing 19a stationarily mounted in the upper central portion of the bellows 19 and resting in a non-fixed manner on the bottom of the tubing 19a.
The amplifying mechanism 15 includes a gear wheel 17 journaled on a base plate 13 and a pinion (not shown) integral and coaxial with a pointer 16 and adapted to mesh with the gear wheel 17. The gear wheel 17 is rotated through a predetermined angle as the rod 14 is axially shifted by the expansion or contraction of the bellows 19 to actuate the amplifying mechanism 15 and swing the pointer 16 a corresponding amount.
In the conventional tonometer illustrated in FIG. 1, the bellows 19 is mounted within a protruding bottom portion 12 of a casing 10 while the base plate 13 is mounted within a main portion 11 of the casing 10. The bellows 19 is fixedly connected with the nozzle portion 18 to the lowermost bottom wall 12a of the protruding bottom portion 12 of casing 10 by means of a ring nut 18a. The base plate 13 is fixedly connected centrally to the casing main portion 11 by means of a screw or similar threaded fastener 13a.
In the tonometer of the prior art illustrated in FIG. 1, however, the respective components described above are made of different materials. More particularly, the casing 10 is made of a material (usually of zinc die casting having a relatively high coefficient of thermal expansion) which is different from the materials of the base plate 13 and the rod 14. For this reason, the inherent zero point and sensitivity of the instrument will be altered when changes in the ambient temperature occur so that such temperature changes, in consequence, cause instrumental error.
Attempts have been made to overcome the problem described above. Thus, it has been proposed that the error resulting from ambient temperature changes can be reduced or eliminated by reducing the coefficient of thermal expansion of the casing 10 by molding the casing 10 of aluminum die casting having a relatively low coefficient of thermal expansion or by providing an inner structure comprising an integral frame-like casing formed of steel while the base plate, amplifying mechanism and other components are mounted within an outer frame formed of synthetic resin, such as plastic. Although this proposal has indeed reduced the thermal expansion of the casing and has correspondingly reduced the influence derived from the thermal expansion of the casing, this solution has not proven to be entirely satisfactory.
Moreover, use of the same material for all of the components of the tonometer in order to alleviate the problem of thermal expansion is not a practical solution. Thus, the types of materials from which the components of the tonometer can be formed is clearly limited by other factors, such as durability, performance and manufacturing cost. For example, zinc die casting which is suitable for molding and plating of the casing 10 is not especially suited for use in the construction of the other tonometer components.